This invention relates to ceramic packages and other housings used for enclosing or housing active and passive electronic devices, e.g., transistors, resistors, and capacitors, and in particular integrated circuits, such as MMICs or monolithic microwave integrated circuits. The invention is more particularly concerned with a construction of ceramic packages that are adapted to be surface-mounted onto a printed circuit board. That is, the package itself is to be soldered or bonded directly to microstrips or other metal traces on the printed circuit board. The invention is also concerned with minimizing inductive impedance as much as possible in the signal paths into and out of the integrated circuit device that is contained within the ceramic package.
Ceramic packaging is commonly used housing for small integrated circuits and other solid-state electronic devices. Co-fired ceramic enclosures are often employed as packages, and the advantages of them are well known. The integrated circuit is typically housed in or on a ceramic substrate, or in a cavity within the substrate, and metallized feed-throughs connect the integrated circuit with the outside of the package. Many packages have a metal base or core that serves as a ground plane, and some packages incorporate a metal slug for improved heat removal. Commonly, thick film or screen printing techniques are used to form the traces. These can be applied as a conductive ink to the green tape, and co-fired with the green tape. Alternatively, the traces can be printed onto fully-fired ceramic substrates, e.g., stacked prefired ceramic. Some packages are formed of a molded polymer for the package wall, with an embedded lead frame. Metal leads, typically Kovar(trademark) or copper ribbons, are soldered or brazed onto the metal traces to provide connections from the package to exterior circuitry. Printed feed-throughs have to be matched to a transmission line, which is typically a microstrip that is printed on the circuit board, and wire bonds, ribbon bonds, or leads at this point create an inductance, which can result in undesirable insertion loss, especially at higher frequencies.
For high frequency circuit packaging of digital or analog circuits, e.g., in the microwave and millimeter wave frequencies, controlled impedance feed-throughs and interconnections are provided to control signal reflections and losses to acceptable levels. Most components of this type are designed for nominal 50 ohm impedance for their high-frequency connections. To realize feed-throughs of a specified characteristic impedance requires careful design of conductor geometry, which requires consideration of dielectric thickness and permittivity, as well as ground geometry. At high frequencies, many feed-through structures will support higher order modes of transmission and resonances, and have dispersive propagation degrading characteristics (i.e., frequency-dependent characteristic impedance and propagation velocity) thereby degrading signal fidelity. To reduce or minimize these deleterious effects, thinner dielectric substrates are used. For frequencies of 40 GHz or above, substrate thickness of 0.010 inches or less is desirable. Substrate thickness refers to the separation between the ground plane and the microstrip transmission line or lines, i.e., conductive I/O signal traces. For typical packages made of co-fired alumina, frequently referred to as HTCC, with a dielectric constant of about 9, the metal trace widths are only about 0.01 inches in order to maintain the nominal 50 ohm characteristic impedance.
Keeping in mind that the traces on a well-designed microwave package are themselves controlled impedance transmission lines, and that the interconnect substrate can typically be a printed circuit board of appropriate material and thickness for high frequency operation, the lead lengths between the package and the printed circuit board traces should be kept extremely short in order to achieve broadband performance (typically, DC to 40 GHz or more) with low-loss interconnection.
Consequently, the industry has sought a package arrangement that permits the transmission line conductors on the package to be connected to the microstrip conductors on the printed circuit board with a minimum of signal loss. It has also been a desire to simplify the job of installation of the packaged integrated circuits onto the printed circuit board. A surface-mount construction, wherein all package signal and ground connections lie in the same plane, would appear to address these issues, but no satisfactory surface mount package has appeared for this application. Typical surface-mount packages employ metallic vias through a dielectric layer (preferably, ceramic) which transfers both signal and ground connections from the device to the package mounting surface.
The typical surface mounted device (SMD) package is attached to the upper surface of a printed circuit board (preferably using solder) wherein electrical grounds (both DC and RF) and much of the heat generated by the packaged device, are carried to chassis, i.e., cold plate and chassis ground, by a number of metallized vias in the printed circuit board. Controlled impedance interconnects, which are a necessity for millimeter wave operation, require attention not only to signal trace geometry but also to ground current path geometry. Prior art surfacemount packages demonstrate a degraded performance above about 20 GHz due to the inadequate ground integrity of metallized vias, particularly the combination of series connected ground vias through the package base plus the printed circuit board. This ground-integrity issue manifests itself in significant resonance effects, which produce rapid increases in signal loss, reflection, and radiation above a threshold frequency for the package/circuit board combination.
Accordingly, it is an object of the present invention to provide a ceramic package that avoids the drawbacks of the prior art, and in particular which may be surface mountable so as to avoid problems that may arise from conventional package designs.
It is a further object to provide a ceramic package in which input and output pads on the exterior of the package can be directly connected to the associated outside circuitry, e.g., with solder or with an isotropic conductive adhesive, so as to avoid inductive losses that are characteristic of wire or ribbon bonds, or leads in the signal connections.
It is a further object to avoid inductive losses and resonances caused by package via grounding and printed circuit board grounding.
It is another object to create a surface mount ceramic package that facilitates automated installation of the package onto the associated printed circuit board.
It is yet another object to enhance thermal conduction from the packaged device to the system heat sink using package base materials that possess high thermal conductivity.
In accordance with an aspect of the present invention, a ceramic package for a microwave or millimeter wave or high density integrated circuit, is adapted to be mounted directly onto a printed circuit board on which there are metallized traces. The package has a flat core formed of a material that is thermally and electrically conductive. This may be Kovar, Cu/W, Cu/Mo, or another suitable metal, or may be a ceramic material that is metallized on its upper and lower surfaces and may contain vias to connect the upper and lower surfaces, or a ceramic base which is metallized on all sides, or a semiconductor such as silicon which is doped for electrical conductivity. The ceramic material can be applied by a low-temperature co-fired ceramic technique, in which green tape is cut and stacked onto the core or base layer. The ceramic material can be considered to have a first ceramic layer applied onto the core of Kovar, Cu/W, Cu/Mo, or other suitable material, and a second ceramic layer atop the first layer. The first ceramic layer, which is disposed on the core, is formed with a cavity therein, and the high density integrated circuit, in the form of a semiconductor die, lies on the core with the first ceramic layer surrounding the cavity. Metallized feed-throughs are formed on an upper surface of the first ceramic layer, each feed-through extending to an inner metallized pad adjacent the cavity. The second ceramic layer is disposed on the upper surface of said first ceramic layer and encircles the cavity, so that the second layer covers the feed-throughs but leaves the inner pads available for bonding to the die.
Outer metallized pads are formed on a lower surface of the first ceramic layer for connecting electrically onto the metallized traces of the printed circuit board, while metal vias in the first ceramic layer, or other suitable conductor means, connect the outer metallized pads to respective interior contact pads for the feed-throughs. In order to accommodate these downward-facing outer conductive pads, the core is formed with one or more peripheral cutouts or accesses that permit access to the lower surface of the first ceramic layer at the locations of the outer pads. This construction permits the outer pads to be connected directly to the printed circuit board metal traces.
In one possible embodiment, the lower surface of the first ceramic layer is substantially in the plane of the upper surface of the core, such that the outer pads are elevated somewhat above the bottom side, i.e., lower surface, of the core. In another possible embodiment, ceramic material is present within the cutout portions of the core, that is, a portion of the first ceramic layer where the outer pads are located extends down into one or more of the cutouts. Another alternative embodiment employs a continuous ring of ceramic material around the perimeter of the core, with the ring being substantially coplanar with the bottom face of the core. Yet another embodiment can employ a core and a one-layer wall structure, with interconnects; with a dome lid or cover being used over the packaged device. The wall structure in this embodiment may be ceramic, dielectric polymer, ceramic/polymer combination, or other suitable material.
The packaged integrated circuits can be installed on the printed circuit board by placing the package(s) at the appropriate location(s) and passing the printed circuit board through a reflow furnace. Alternatively, an isotropic conductive adhesive, or a solder-ball system may be used to attach the connection pads to the printed traces on the board.
The above and many other objects, features, and advantages of this invention will become apparent from the ensuing description of selected preferred embodiments, which are to be considered in connection with the accompanying Drawing.